This invention relates generally to methods and apparatus for CT image reconstruction, and more particularly to methods and apparatus for utilizing more than one scan for reconstruction of CT images that are particularly advantageous for cardiac CT imaging.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. Each discrete attenuation measurement in a view corresponding to a given detector angle xcex3 is referred to as being obtained from a detector xe2x80x9cchannel.xe2x80x9d A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In known cardiac CT studies, two scans are performed, one with and one without contrast injection. In these studies, a full field of view (FOV) scan is performed both times, because a significant amount of time elapses between the two scans, and patient motion can occur. In addition, breath-hold levels of the two scans may be different. It would be desirable to minimize the radiation dose to a patient when performing such a study.
There is therefore provided, in one embodiment of the present invention, a method for imaging a volume of a patient with a computed tomographic (CT) imaging system having a radiation source and a detector array opposing one another on a rotating gantry and configured so that radiation from the radiation source passing through a patient impinges on the detector array. The method includes steps of: scanning a volume of a patient with a first, full field of view (FOV) scan to acquire first projection data; scanning a smaller volume of the patient with a second, restricted FOV scan to acquire second projection data; estimating an amount of shift between the first projection data and the second projection data resulting from patient movement; and blending first projection data with second projection data in accordance with the estimated amount of shift to estimate projections of the second scan.
Embodiments of the present invention minimize the radiation dose to a patient when performing cardiac studies in that the second scan can be performed using a radiation beam collimated to restrict its coverage to a lesser portion of the patient.